子集和数问题——回溯法

 已知n个正数：wi, 1<=i<=n, 和M。要求找出{wi }的所有子集使得子集内元素之和等于M。例如：

       n=4,     (w1,w2,w3,w4)=(11,13,24,7)，M=31   则满足要求的子集是(11,13,7)和（24,7）。

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package com.lanxi.demo1;   public class Subset {     static int M,n;     static int[] w=new int[100];     static int[] x=new int[100];     public static void SumOfSub(int s, int k, int r)//s:当前和数 k:当前个数 r:总数和     {         int i;         x[k]=1;         if(s+w[k]==M) //子集找到         {             System.out.print("answer:");             for(i=1; i<=k; i++)                 if(x[i]==1) System.out.print(w[i]+" ");             System.out.println("");         }else if(s+w[k]+w[k+1]<=M)             SumOfSub(s+w[k], k+1, r-w[k]);                   if(s+r-w[k]>=M && s+w[k+1]<=M)         {             x[k]=0;             SumOfSub(s, k+1, r-w[k]);         }     } public static void main(String arg[])     {         int s,k,r;         n=4;         w[1]=7;         w[2]=11;         w[3]=13;         w[4]=24;         M=31;                   s=0;         k=1;         r=55;                   SumOfSub(s, k, r);     }   }